An equilibrium constant for dissociation. Thus, for the reaction: A+B = C, at equilibrium Kd = (A) (B) /(C). Dimension: moles per litre in this case. Kd is the reciprocal of Ka. In general the concept of Kd is more readily understood than that of Ka; for example, in considering the conversion of A to C by the binding of ligand B, the Kd = B when A = C. Thus Kd is equal to the ligand concentration which produces half-maximal conversion (response).
Dictionary of molecular biology. 2004.